Heat engines, such as combined heat and power (CHP) appliances that are based on an organic Rankine cycle (ORC) module, are known. Heat engines of this kind employ a positive displacement device, such as a scroll-expander, connected to a generator, such as a permanent magnet generator, in a single unit. Such CHP appliances may replace conventional gas boilers to provide heat for central heating and hot water, with electricity produced as a by-product.
An example of a simple known ORC 10 heat engine is shown schematically in FIG. 1A. The ORC has a working fluid circuit 12 that includes an evaporator 14 acting as a heat source for heating a working fluid circulating around the working fluid circuit 12, a positive displacement expander-generator 16, a condenser heat exchanger 18 acting as a heat sink for cooling the working fluid and a pump 20. Each of evaporator heat exchanger 14, expander-generator 16, condenser 18 and pump 20 are fluidly connected in series in the working fluid circuit 12. The expander-generator 16 has an inlet in fluid communication with the evaporator 14, and an outlet in fluid communication with the condenser 16. The pump 20 is disposed in the working fluid circuit 12 between the condenser 18 and the evaporator 14 but on the opposite side of the condenser 18 to the expander-generator 16.
In steady state operation, the working fluid is evaporated in the evaporator 14 at high pressure (pressure P1) and temperature T1. The evaporator 14 receives an input of heat Qin and does work Win to raise the temperature of the working fluid to temperature T1. The evaporated gas phase fluid is then expanded through the expander-generator 16 thus producing electrical energy, We. The gas exits the expander-generator 16 at a lower pressure P2 and temperature T2 and is then condensed back to the liquid phase in the condenser 18 where the latent heat of condensation is given up to a cooling circuit (not shown). The condenser 18 receives a coolant so as to remove energy Wout and heat Qout from the working fluid. The low temperature T2′ and low pressure P2 liquid phase working fluid is then pumped back to the evaporator at high pressure P1 by the pump 20, thus completing the cycle.
Upon starting the ORC heat engine 10 of FIG. 1A, heating Qin and cooling Qout is supplied to the evaporator 14 and condenser 18, respectively, and the pump 20 is operated to provide the high pressure P1 and flow of working fluid into the evaporator 14. Initially, the expander-generator 16 is not rotating so there is no flow of working fluid around the working fluid circuit 12. The expander-generator 16 does not begin to rotate when the pump 20 begins to run due to seal and bearing friction together with the mass of the generator parts. Additionally, a negative pressure differential begins to form across the expander-generator 16 as the expander tries to expand pockets of gas that have equalised with the low pressure working fluid when at rest.
To overcome this initial “stiction”, a large initial inlet pressure is required to start the rotation. This initially high starting pressure is supplied by the pump 20. However, since the expander-generator 16 is not rotating initially, there is very little working fluid flowing through the pump 20. This situation is detrimental to the lifetime and performance of the pump 20 since the pump 20 can overheat and lubrication therein can be reduced.
Another undesirable situation that may arise at start-up is the pump 20 beginning to run dry. This might happen where the non-rotating expander-generator 16 acts as a blockage along the working fluid circuit 12 and the pump 20 works to displace working fluid towards the evaporator 14. Without sufficient circulation of working fluid, the entire volume of working fluid could be pumped into the evaporator 14, causing the pump 20 to run dry, thereby increasing pump wear and reducing its lifetime.
In order to successfully replace a conventional gas boiler from the operator's perspective, an ORC heat engine, such as a CHP appliance, should be able to operate across a range of temperatures and heat demands, and should be able to be turned on and off in the same manner as a conventional gas boiler system.
It is an object of the present invention to provide an ORC heat engine that improves over prior art ORC heat engines, by having, for example, an improved start-up time, improved component lifetime and performance, or increased operational efficiencies.